This application claims the priority of Korean Patent Application No. 2003-38319, filed on Jun. 13, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a high efficiency projection system and a method of forming a color image using the same. More particularly, the present invention relates to a projection system which uses a plurality of light valves, thereby accomplishing a wide color gamut and color temperature and improving light efficiency, and a method of forming a color image using the same.
2. Description of Related Art
Projection systems are divided into a three-panel type and a single-panel type according to the number of light valves which control light emitted from a high-output lamp light source to be turned on or off in each pixel. A single-panel projection system has a smaller optical structure than a three-panel projection system but has only ⅓ of the light efficiency of the three-panel projection system because it splits white light into red (R), green (G), and blue (B) light using a sequential method. Accordingly, research and development have been performed to increase light efficiency of single-panel projection systems.
A general single-panel panel projection optical system splits light emitted from a white light source into R, G, and B light using a color filter, sequentially transmits the R, G, and B light to a light valve, and operates the light valve according to the order of color to form an image. Since such a single-panel optical system uses colors sequentially, the light efficiency of the single-panel optical system is only ⅓ of that of a three-panel optical system. Approaches for increasing light efficiency of a single-panel projection system have been developed, and a scrolling method has been proposed as one of these approaches. According to the color scrolling method, white light is split into R, G, and B light, and the R, G, and B light is simultaneously transmitted to different positions on a light valve. In addition, because an image can be formed only when all of the R, G, and B light reaches each pixel, color bars are periodically moved in a cycle using a particular method.
A conventional single-panel scrolling projection system is disclosed in U.S. patent Publication No. 2002/191154 A1. As shown in FIG. 1, in the conventional single-panel scrolling projection system, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarization converter 105 and is split into R, G, and B light by first through fourth dichroic filters 109, 112, 122, and 139. For example, the R and B light is transmitted by the first dichroic filter 109 and proceed on a first optical path L1, and the G light is reflected by the first dichroic filter 109 and proceeds on a second optical path L2. The R and B light proceeding on the first optical path L1 is split again by the second dichroic filter 112 such that the R light is transmitted by the second dichroic filter 112 and goes straight on the first optical path L1 and the G light is reflected by the second dichroic filter 112 and proceeds on a third optical path L3.
First through third prisms 114, 135, and 142 are rotatably disposed on the first through third optical paths L1, L2, and L3, respectively. Light radiated from the light source 100 is split into R, G, and B light, and the R, G, and B light is scrolled by the corresponding first through third prisms 114, 135, and 142, respectively. The first through third prisms 114, 135, and 142 rotate at a constant speed so as to scroll R, G, and B color bars. The G and B light respectively proceeding on the second and third optical paths L2 and L3 is respectively transmitted and reflected by the third dichroic filter 139 and thus mixed. Thereafter, the R, G, and B light is mixed by the fourth dichroic filter 122 and then transmitted by a polarizing beam splitter 127 to a light valve 130. The light valve forms a color image.
A focusing lens 107 is disposed next to the polarization converter 105, and lenses 110, 117, 131, 137, 145 for optical path compensation are disposed on the first through third optical paths L1, L2, and L3. A focusing lens 120 is disposed between the first dichroic filter 112 and the fourth dichroic filter 122, and a focusing lens 140 is disposed between the third dichroic filter 139 and the fourth dichroic filter 122. A focusing lens 124 and a polarizer 125 are disposed on an optical path between the fourth dichroic filter 122 and the polarizing beam splitter 127. Optical path converters, for example, reflecting mirrors 118 and 133 are disposed on the first and third optical paths L1 and L3, respectively, to change an optical path of light.
FIG. 2 illustrates a procedure in which the R, G, and B color bars are scrolled by the rotations of the first through third prisms 114, 135, and 142. FIG. 2 shows how the color bars formed on a surface of the light valve 130 are periodically cycled when the first through third prisms 114, 135, and 142 corresponding to respective colors are rotated in synchronization with one another. When the R, G, and B color bars are moved in a single cycle, a color image of a single frame is formed.
The light valve 130 processes an image signal for each pixel to form an image, and the image is enlarged and projected onto a screen by a projection lens unit (not shown).
In conventional single-panel systems forming a color image using the scrolling method, it is difficult to implement multi channels by forming three or more color bars. When light radiated from a light source is split into three or more colors in conventional single-projection systems, the etendue of an optical system increases, which makes the formation of an optical system difficult.
The term “etendue” (E) refers to a conserved physical quantity measuring the dimensions of light in an optical system. The etendue is obtained using an area of a target, the etendue of which is to be measured, and the square of a sine value of half of an angle at which light is incident onto or output from the area of the target and can be expressed as the following equation.
  E  =            π      ⁢                          ⁢      A      ⁢                          ⁢                        sin          2                ⁡                  (                      θ                          1              /              2                                )                      =                  π        ⁢                                  ⁢        A                              (                      2            ⁢                          F              /              No                                )                2            
Here, F/No indicates an F number of a lens, and a relationship
      sin    ⁡          (              θ                  1          /          2                    )        =      1          (              2        ⁢                  F          /          No                    )      is accomplished. With an increase in an etendue, an optical system increases in volume and becomes complicated in structure.
Since it is difficult to form three or more color bars in conventional single-panel projection systems using color scrolling, increasing a color gamut is limited, and implementation of various colors is difficult.